Diversité et dégâts associés aux principaux ravageurs de Vernonia calvoana Hook,1868 (Asteraceae) dans la ville de Yaoundé, Cameroun

Abstract

Vernonia calvoana, or sweet bitterleaf in English and ndolè in the Duala language, is a forest vegetable from Cameroon. It is prized for its culinary and medicinal properties. However, its cultivation faces various challenges such as pests and/or pathogenic organisms. The aim of this thesis was to gain a better understanding of the bio-ecology of the main pests and field beneficials of this plant. The specific objectives were to determine: (1) the diversity of the invertebrate fauna associated with V. calvoana and, (2) the damage inflicted on V. calvoana. Sampling was carried out in Yaoundé from May 2017 to April 2019 at the Nkolbisson and Nlong-mvolye sites, Mfoundi Department, Centre Region (Cameroon). In the field, invertebrate communities were collected using manual harvesting on the one hand, and on the other hand monitored for three types of leaf damage (aggravated perforations, rotting and rolling) singly or in association on the white and purple forms of the host plant. A total of 13929 invertebrate specimens were collected, grouped into two phyla, four classes, 12 orders, 47 families, 92 genera and 168 morphospecies. The invertebrate population at Nkolbisson was more diverse (12 orders, 44 families, 86 genera and 152 species) than that of Nlong-mvolye (10 orders, 22 families, 33 genera and 42 species). Overall, Formicidae (6863 individuals), the Aphididae (5155 individuals), the Tettigometridae (651 individuals), Pentatomidae (388 individuals), Coccinellidae (288 individuals), the Pyrgomorphidae (143 individuals) and the Chrysomelidae (101 individuals) with a representation rate of 97.56% of the total number of individuals collected; the species Uroleucon(Uromelan) compositae (37.01%) was abundant; Diplomorium longipenne (12.59%) and Pheidole megacephala (23.01%) were not very abundant. Healthy plants (30.93%) were abundant, while those with those with leaves: perforated and rotten (18.56%), perforated (14.76%), with worsened perforations and rotten (12.76%), with worsened perforations and rolled (1.94%), rotten (1.78%), rolled (0.33%) and finally rotten and rolled (0.28%) were scarce or rare. Pollinators, phyllophages, sap-suckers, parasitoids and predators were the trophic groups sampled. The sap-sucking and phyllophagous guilds were the most abundant (51.41%) and the most diverse in terms of species (76.19%). Hemiptera (sap-sucking) species accounted for 73.41% of the total number of the guild and phytophagous species. Uroleucon compositae was the main pest of V. calvoana in Yaoundé. These results show that the site is the most important factor in the cultivation of V. calvoana than the level of maintenance of the plots and the number of rows. However, insecticide treatments are still necessary to optimise yields in Yaoundé.